Wondering if any of our bike gurus on here can answer this. 2007 Trek 1500 9 spd 105's. Don't know if that 28 ring will work with that rd. Also, wonder if chain would have to have more links. Have searched a lot but no luck yet.

Would love to have 1 more tooth for Mt Mitchell. Wouldn't mind an 11 either. I don't mind any large gaps in gearing either.

It depends on the derailleur. If it's long cage then not a problem. If it's short cage then it will be on the very outside of it's recommended capability. It depends on what your cassette is now on whether you'll need a longer chain.

A couple guys from my bike shop rode Mt. Mitchell last year and one of them changed his RD to a long cage and used a mountain bike cassette, 11-32 or 11-34.

I run a short cage rear with a 28t cassette and a standard crank.
As the step on a compact crank is larger, you should have no problems in the small ring.
If you plan on using your 50 ring with the 28, you are going to knacker your drive train from cross chaining just like you would with the 25!

If its the one that came on your bike and it came with a 12-25 and a compact crank it will be a short cage.
In Ultegra I have a 6700SS and a 6600GS rear, 66 is the older series, 67 the newer, SS is the short cage and GS is the medium drop.
I use the GS on my commuter/CX bike as I have a compact crank and one of my cassettes is a 13-32 for the steep Welsh trails.
My Tri bike has the 6700SS and works perfectly with a 12-28 cassette.

If its the one that came on your bike and it came with a 12-25 and a compact crank it will be a short cage.
In Ultegra I have a 6700SS and a 6600GS rear, 66 is the older series, 67 the newer, SS is the short cage and GS is the medium drop.
I use the GS on my commuter/CX bike as I have a compact crank and one of my cassettes is a 13-32 for the steep Welsh trails.
My Tri bike has the 6700SS and works perfectly with a 12-28 cassette.

A compact crank and a 11/12-28 should get you up anything paved.

It came with a 12-25 and a regular 53/39 crank.

Yep, 28 will be more than adequate, even for the sufferfest that is Mt Mitchell.